Method of fabricating an SOI wafer and SOI wafer fabricated by the method

ABSTRACT

There is disclosed a method of fabricating an SOI wafer in which a bond wafer to form a SOI layer and a base wafer to be a supporting substrate are prepared; an oxide film is formed on at least the bond wafer; hydrogen ions or rare gas ions are implanted in the bond wafer via the oxide film in order to form a fine bubble layer (enclosed layer) within the bond wafer; the ion-implanted surface is brought into close contact with the surface of the base wafer; and then heat treatment is performed to separate a thin film from the bond wafer with using the fine bubble layer as a delaminating plane to fabricate the SOI wafer having an SOI layer; and wherein deviation in the thickness of the oxide film formed on the bond wafer is controlled to be smaller than the deviation in the ion implantation depth, and the SOI wafer fabricated thereby. There is provided an SOI wafer which has an SOI layer having improved thickness uniformity.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an SOI (silicon on insulator)structure wafer obtained by a method wherein an ion-implanted wafer isbonded to another wafer and a portion of the ion-implanted wafer isdelaminated (separated) to provide an SOI wafer (called a smart-cutmethod). More particularly, the present invention relates to a method offabricating an SOI wafer having an active SOI layer which is excellentin thickness uniformity, and an SOI wafer having an active SOI layerwhich is excellent in thickness uniformity.

[0003] 2. Description of the Related Art

[0004] Recently, a method of fabricating an SOI wafer in which anion-implanted wafer is bonded to another wafer and a portion of theion-implanted wafer is delaminated to provide an SOI wafer (hereinafter,occasionally referred to as an ion implantation and delamination method)has attracted attention. In this method, as shown in FIGS. 3(a) to (h),an oxide film is formed on the surface of at least one of two siliconwafers (FIG. 3(b)), which are a bond wafer 2 to form an SOI layer and abase wafer 1 to be a supporting substrate (FIG. 3(a)); hydrogen ions orrare gas ions are implanted into the bond wafer 2 in order to form afine bubble layer (enclosed layer) 4 within the bond wafer (FIG. 3(c));the ion-implanted wafer is superposed on the base wafer 1 such that theion-implanted surface comes into close contact with the surface of thebase wafer 1 via the oxide film (FIG. 3(d)); heat treatment is thenperformed to delaminate a portion of the bond wafer 2 as a thin filmwith using the fine bubble layer as a delaminating plane (FIG. 3(e));heat treatment is further performed to firmly bond the wafers to eachother (FIG. 3(f)), which are then subjected to mirror polishing (FIG.3(g)) to thereby obtain an SOI wafer (FIG. 3(h)) (See Japanese PatentApplication Laid-Open (kokai) No. 5-211128).

[0005] The above-mentioned method can be roughly classified into twomethods depending on the wafer on which an oxide film is formed. In oneof them, Method A, the oxide film is formed on the base wafer 1 as shownin FIG. 3(A), and in the other of them, Method B, it is formed on thebond wafer 2 as shown in FIG. 3(B). Method B wherein the oxide film isformed on the bond wafer in advance has been performed mainly.

[0006] Because, the depth of ion implantation varies widely due to achanneling effect, when the oxide film is not formed on the bond waferin which ions are to be implanted, and as a result, thickness uniformityof the SOI layer may be lowered.

[0007] “The channeling effect” means herein a phenomenon wherein ionsimplanted parallel to the crystal axis of the crystalline material passzigzag between the atoms of the crystal. In that case, the deviation inthe ion implantation depth is larger, compared to the case that the ionsare implanted nonparallel to the crystal axis. The phenomenon is apt tooccur particularly in a silicon wafer, since the surface thereof isprocessed in a certain orientation (for example, <100>). Accordingly, itis preferable to form the oxide film on the wafer to suppress thechanneling effect in that case.

[0008] Another reason why the oxide film should be formed on the bondwafer is that the oxide film previously formed on the bond wafer maysuppress diffusion of impurities incorporated in the bonded surface(boron in atmosphere or metal impurities due to ion implantation) intothe active layer (SOI layer), and therefore, degradation ofcrystallinity of the SOI layer and electronic characteristics can beprevented.

[0009] When the channeling effect is not caused, deviation (standarddeviation) σ of the ion implantation depth in the ion implantation anddelamination method can be 0.4 nm. Namely, 3σ=1.2 nm can be achieved, sothat almost all of ions can be implanted at an intended depth ±1.2 nm.Accordingly, it is inferred that a super thin SOI wafer having goodthickness uniformity of an intended thickness ±1.5 nm or less can beobtained according to the ion implantation and delamination method.

[0010] However, when the oxide film is formed on the bond wafer in whichions are to be implanted because of the above-mentioned reasons, thereis a deviation also in the thickness of the formed oxide film, and theimplantation depth of ions implanted in the silicon through the oxidefilm is also affected thereby.

[0011] For example, when a thickness of a buried oxide layer of an SOIwafer needs to be 400 nm, a standard deviation σ in the thickness of theoxide layer will be 2.0 nm at the smallest under oxidizing condition ingeneral mass-production. Even if the oxidizing condition is preciselycontrolled without considering productivity, σ is around 1.0 nm at thesmallest. Accordingly, a thickness uniformity of an SOI layer of aconventional SOI wafer produced with forming an oxide film on a bondwafer is around an intended thickness ±3 nm at the smallest.

SUMMARY OF THE INVENTION

[0012] The present invention has been accomplished to solve theabove-mentioned problems, and a major object of the present invention isto provide a method of fabricating an SOI wafer that has an SOI layerwith significantly improved thickness uniformity, by holding aninfluence of deviation in thickness of an oxide film formed on a bondwafer in the SOI wafer on thickness uniformity of the SOI layer to theminimum, and the SOI wafer.

[0013] To achieve the above mentioned object, the present inventionprovides a method of fabricating an SOI wafer in which a bond wafer toform a SOI layer and a base wafer to be a supporting substrate areprepared; an oxide film is formed on at least the bond wafer; hydrogenions or rare gas ions are implanted in the bond wafer via the oxide filmin order to form a fine bubble layer (enclosed layer) within the bondwafer; the ion-implanted surface is brought into close contact with thesurface of the base wafer; and then heat treatment is performed toseparate a thin film from the bond wafer with using the fine bubblelayer as a delaminating plane to fabricate the SOI wafer having an SOIlayer; and characterized in that deviation in the thickness of the oxidefilm formed on the bond wafer is controlled to be smaller than thedeviation in the ion implantation depth.

[0014] As described above, in the method of fabricating an SOI wafer inwhich an oxide film is formed on a bond wafer in advance, if thedeviation in the thickness of the oxide film formed on the bond wafer iscontrolled to be smaller than the deviation in the ion implantationdepth, the influence of the deviation in the thickness of the oxide filmon the thickness uniformity of the SOI layer can be held to the minimum,and thus the SOI wafer wherein thickness uniformity of the SOI layer isimproved can be fabricated.

[0015] The term “deviation” herein means a standard deviation.

[0016] To achieve the above mentioned object, the present invention alsoprovides a method of fabricating an SOI wafer in which a bond wafer toform a SOI layer and a base wafer to be a supporting substrate areprepared; an oxide film is formed on at least the bond wafer; hydrogenions or rare gas ions are implanted in the bond wafer via the oxide filmin order to form a fine bubble layer (enclosed layer) within the bondwafer; the ion-implanted surface is brought into close contact with thesurface of the base wafer; and then heat treatment is performed toseparate a thin film from the bond wafer with using the fine bubblelayer as a delaminating plane to fabricate the SOI wafer having an SOIlayer; and characterized in that a thickness of the oxide film formed onthe bond wafer is defined so that deviation in the thickness of theoxide film formed on the bond wafer may be smaller than the deviation inthe ion implantation depth.

[0017] As described above, in the method of fabricating the SOI wafer inwhich an oxide film is formed on a bond wafer in advance, when thethickness of the oxide film formed on the bond wafer is defined so thatdeviation in the thickness of the oxide film formed on the bond wafermay be smaller than the deviation in the ion implantation depth, theinfluence of the deviation in the thickness of the oxide film on the ionimplantation depth can be held to the minimum, and thus the SOI waferwherein thickness uniformity of the SOI layer is improved can befabricated.

[0018] Preferably, an oxide film is previously formed on the base waferwhich is to be bonded to the bond wafer, and the thickness of the oxidefilm formed on the base wafer is defined so that it can form the buriedoxide layer with a desired thickness in the SOI wafer together with theoxide film formed on the bond wafer.

[0019] As described above, when the oxide film is previously formed onthe base wafer, and the thickness of the oxide film formed on the basewafer is defined so that it can form the buried oxide layer with adesired thickness in the SOI wafer together with the oxide film formedon the bond wafer, the buried oxide layer having a desired thickness caneasily be obtained, and thus problem of insufficient thickness of theoxide layer is not caused, and the influence of the deviation in thethickness of the oxide film of the bond wafer on the thicknessuniformity of the SOI layer can be held to the minimum.

[0020] The thickness of the oxide film formed on the bond wafer ispreferably 10 to 100 nm.

[0021] Although the deviation in the thickness of the oxide film and thedeviation in the ion implantation depth vary depending on a usedapparatus, conditions for ion implantation and oxidizing conditions forformation of the oxide film, in order to make the deviation in thicknessof the oxide film smaller than the deviation in ion implantation depthwhen using an apparatus used in mass production, the thickness of theoxide film is preferably 100 nm or less. On the other hand, at least 10nm of the thickness of the oxide film is necessary in order to preventthe channeling effect described above. Accordingly, the thickness of theoxide film is preferably 10 nm or more.

[0022] The present invention also provides an SOI wafer produced by theabove mentioned methods, wherein the thickness uniformity of the SOIlayer is high even when the buried oxide layer is thick.

[0023] The present invention also provides an SOI wafer produced bybonding two wafers according to the ion implantation and delaminationmethod, which has a bonded surface in the buried oxide layer, or betweenthe buried oxide layer and the base wafer. In the wafer, the thicknessuniformity of the SOI layer is high as ±1.5 nm or less. Accordingly,device characteristics of the device fabricated therefrom are improved,and freeness in designing of the device is large.

[0024] As described above, according to the present invention, influenceof the deviation in the thickness of the oxide film on the bond wafer onthe deviation in the active SOI layer is small, even in the SOI waferwhich requires thick buried oxide layer. Therefore, the SOI layer havingthe SOI layer with quite excellent thickness uniformity can befabricated. Furthermore, device characteristics and freeness of thedevice design can be improved due to decrease of the deviation of thethickness of the SOI layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIGS. 1(a)-(i) is a flowchart showing an example of an SOI-waferfabricating process according to the method of fabricating an SOI waferof the present invention.

[0026]FIG. 2 is a graph showing a result of measurement of a deviationin a thickness of an oxide film and a deviation in an ion implantationdepth, which are shown in the same graph. The line (a) shows a relationbetween the thickness of the oxide film and the deviation therein. Theline (b) shows a relation between the ion implantation depth and thedeviation therein.

[0027] FIGS. 3(a)-(h) is a flowchart showing an example of an SOI-waferfabricating process according to an ion implantation and delaminationmethod. (A) shows a method wherein an oxide film is formed on a basewafer. (B) shows a method wherein an oxide film is formed on a bondwafer.

DESCRIPTION OF THE INVENTION AND A PREFERRED EMBODIMENT

[0028] The present invention will be further described below in detail,but is not limited thereto.

[0029] In fabrication of the SOI wafer, formation of the buried oxidelayer having a thickness necessary for the SOI wafer under generaloxidizing condition for mass production may results in far largerdeviation in the thickness of the oxide film formed in the wafer thanthe deviation in the ion implantation depth. Accordingly, the thicknessuniformity of the SOI layer may be significantly affected thereby.

[0030] Generally, the deviation in the thickness of a thicker oxide filmis larger. Accordingly, the inventors of the present invention havethought of making the thickness of the oxide film formed on the bondwafer thin in order to make the deviation in the thickness of the oxidefilm smaller than the deviation in the implantation depth of ionimplantation, and making up for a deficiency in the thickness of theburied oxide layer necessary for the SOI wafer by forming the oxide filmon the base wafer, to obtain a buried oxide layer having a desiredthickness in combination of the oxide film formed on the bond wafer andthat formed on the base wafer.

[0031] For example, when the upper limit of the thickness of the oxidefilm in the case that the deviation in the implantation depth achievedby an ion implanter used for fabrication of SOI wafer is 0.4 nm and thedeviation in the oxide film formed on the bond wafer is 0.4 nm or less,is 100 nm, the thickness of the oxide film of 100 nm or less can beselected as the thickness of the oxide film formed on the bond wafer.

[0032] In that case, when the thickness of the oxide film on the bondwafer is defined to be, for example, 40 nm, and the thickness of theburied oxide layer necessary for SOI wafer is 400 nm, the oxide filmhaving a thickness of 40 nm can be formed on the bond wafer, in whichions are then implanted, and the resulting wafer can be bonded to thebase wafer on which an oxide film having a thickness of 360 nm isformed.

[0033] The relation between the deviation in the thickness of the oxidefilm and the deviation in the ion implantation depth is important forthe present invention. Accordingly, the inventors of the presentinvention studied the deviation in the thickness of the oxide film andthe deviation in the ion implantation depth.

[0034] The deviation in the thickness of the oxide film was studied asfollows. First, each of the oxide films having various thickness wasactually formed on the wafer under a general oxidizing condition formass production. Then, the standard deviation was determined from thethickness distribution in the surface as for each wafer. As a result,the relation between the thickness of the oxide film and the deviationin the thickness of the oxide film was revealed as shown in the line (a)of FIG. 2.

[0035] The deviation in the ion implantation depth was studied asfollows. First, a wafer having no oxide film was prepared in order toavoid the influence of the oxide film. Then, ions are implanted thereinusing an apparatus generally used, with inclining a implanting angle atseveral degree in order to avoid a channeling phenomenon. The resultingwafer was bonded to a wafer on which an oxide film is formed, followedby a heat treatment for delamination at about 500° C. to produce an SOIwafer. The thickness of the resulting SOI wafer was then measured todetermine the distribution thereof in the surface. Plural wafers werefabricated with implanting ions therein at various implantation energy,and were determined as for the thickness distribution in the surface asdescribed above. Thereby, the relation between the ion implantationdepth and the deviation therein was revealed as shown in the line (b) ofFIG. 2.

[0036] Comparing the deviation in the thickness of the oxide film andthe deviation in the ion implantation depth, it is revealed that thedeviation in the thickness of the oxide film shown as black circles inthe line (a) of FIG. 2 increases in proportion to increase of thethickness of the oixde film formed on the wafer. On the other hand, itis revealed that the deviation in the ion implantation depth shown aswhite circles in the line (b) of FIG. 2 does not change a lot, and isapproximately in the range of 0.4 to 0.6 nm, even when the ionimplantation depth increases.

[0037] From the relations mentioned above, an appropriate thickness ofthe oxide film to be formed on the bond wafer can be defined.

[0038] Namely, in the relation in FIG. 2 which shows the relation whenusing the apparatus and the condition generally used for massproduction, the thickness of the oxide film formed on the bond wafer isappropriately 100 nm or less in order to make the deviation in thethickness of the oxide film smaller than the deviation in the ionimplantation depth.

[0039] When the thickness of the oxide film on the bond wafer isthinner, the deviation in the thickness of the oxide film is smaller, sothat the influence of the deviation in the thickness of the oxide filmon the ion implantation depth gets smaller. However, in order to preventchanneling effect which is one of the reasons why the oxide film isformed on the bond wafer, the thickness of the oxide film of at least 10nm is necessary. Accordingly, when the SOI wafer is actually fabricated,the thickness of the oxide film formed on the bond wafer is preferably10 to 100 nm.

[0040] In that case, even when the deviation in the thickness of theoxide film and the deviation in the ion implantation depth are changeddepending on the ion implanter or conditions such as the condition forformation of the oxide film, the deviations therein can be studied asdescribed above, to make the deviation in the thickness of the oxidefilm smaller than the deviation in the ion implantation depth.

[0041] The embodiment of the present invention will be further describedbelow, referring a figure, but is not limited thereto.

[0042]FIG. 1 is a flowchart showing an example of an SOI waferfabricating process according to a method of fabricating an SOI wafer ofthe present invention.

[0043] In Step (a) of the ion implantation—delamination method shown inFIG. 1, two mirror wafers, namely a base wafer 1 to be a base and a bondwafer 2 to form an SOI layer 8, which are suitable for devicespecifications are prepared.

[0044] In Step (b), the bond wafer 2 is subjected to thermal oxidationso as to form on the surface thereof an oxide film 3 having a thicknessof 10 to 100 nm. As described above, if the thickness of the oxide filmis in the range, the deviation therein can be smaller than the deviationin the ion implantation depth, so that thickness of the SOI layer 8 canbe made uniform.

[0045] In Step (c), the base wafer 1 is subjected to thermal oxidationso as to form on the surface thereof an oxide film 5. The thickness ofthe oxide film 5 formed on the base wafer is defined so that it canprovide a buried oxide layer 9 having a desired thickness for the SOIwafer together with the oxide film 3 formed on the bond wafer. If thedesired thickness of the buried oxide layer for the SOI wafer can beobtained only by the thickness of the oxide film 3 formed on the bondwafer, it is not necessary to form the oxide film on the base wafer. Inthat case, Step (c) is omitted.

[0046] In Step (d), hydrogen ions or rare gas ions are implanted intoone surface of the bond wafer 2 having the oxide film formed on thesurface thereof, in order to form a fine bubble layer (enclosed layer) 4which lies in parallel to the surface at a position corresponding to themean depth of ion implantation. The implantation temperature ispreferably 25-450° C.

[0047] In Step (e), the base wafer 1 is superposed on the ion-implantedsurface of the bond wafer 2 in which hydrogen ions or rare gas ions areimplanted, via the oxide film 3, or via the oxide film 3 and the oxidefilm 5, and they are brought in close contact with each other. When thesurfaces of the two wafers are brought into contact with each other atambient temperature in a clean atmosphere, the wafers are bonded to eachother without use of adhesive or the like.

[0048] In Step (f), there is performed a heat treatment for delaminationin which the delamination wafer 6 is delaminated from the SOI wafer 7(composed of the SOI layer 8, a buried oxide layer 9, and the base wafer1) at the enclosed layer 4. The heat treatment is performed at atemperature of about 500° C. or higher in an inert gas atmosphere so asto cause crystal rearrangement and bubble cohesion, and thereby thedelamination wafer 6 is delaminated from the SOI wafer 7.

[0049] In Step (g), the SOI wafer 7 is subjected to heat treatment athigh temperature as a bonding heat treatment in order to improve bondingstrength to a sufficient level, since the bonding strength of the wafersimparted in Steps (e) and (f), the bonding step and the delaminationheat treatment step, is too low to use in a device process. Preferably,this heat treatment is performed, for example, in an inert gasatmosphere at 1050-1200° C. for 30 minutes to 2 hours.

[0050] The Step (f) as a delamination heat treatment step and Step (g)as a bonding heat treatment step may be successively performed.Alternatively, a single heat treatment serving as both of Step (f) as adelamination heat treatment step and Step (g) as a bonding heattreatment step may be performed.

[0051] In step (h), a mirror polishing process called touch polishingwherein a stock removal is very small is then performed to remove acrystal defect layer on a delaminated surface which is a surface of theSOI layer 8 and improve the surface roughness.

[0052] The SOI wafer 7 of high quality having the buried oxide layer 9with a desired thickness and the SOI layer 8 with high thicknessuniformity can be produced (step (i)) through the steps described above.

EXAMPLES

[0053] The following examples and a comparative example are beingsubmitted to further explain the present invention. These examples arenot intended to limit the scope of the present invention.

Example 1

[0054] Two silicon mirror wafers of conductive type p having aresistivity of 20 Ω·cm and a diameter of 150 mm were prepared. Thesewafers were processed through Steps (a) to (f) shown in FIG. 1 todelaminate a portion of the bond wafer, and thereby provide SOI wafers.

[0055] The major process conditions for fabricating SOI wafers were asfollows.

[0056] 1) Thickness of the oxide film on the base wafer: 350 nm;

[0057] 2) Thickness of the oxide film on the bond wafer: 50 nm;

[0058] 3) Deviation in the thickness of the oxide film on the bondwafer: σ=0.25 nm;

[0059] 4) Condition of ion implantation: H⁺ ions, implantation energy:80 keV, implantation dose: 8×10¹⁶/cm²;

[0060] 5) Ion implantation depth: 700 nm (thickness of the SOIlayer+thickness of the oxide film on the bond wafer);

[0061] 6) Deviation in the ion implantation depth: σ=0.4 nm.

[0062] In Example 1, the deviation σ in the thickness of the oxide filmon the bond wafer was small as 0.25 nm, compared with the deviation inthe ion implantation depth (σ=0.4 nm). The thickness of the oxide filmon the bond wafer was measured by spectral reflectance measurement atseveral thousands points in 2 mm pitch on the surface. The conditionswere set so that the thickness of the buried oxide layer may be 400 nm.

[0063] The two wafers were stacked under the above condition (FIG.1(e)), and was subjected to the delamination heat treatment in a N₂ gasatmosphere, at 500° C. for 30 minutes (FIG. 1(f)).

[0064] The deviation in the thickness of the SOI layer of the SOI waferafter the delamination step were determined to evaluate the thicknessuniformity. The thickness of the SOI layer was measured by spectralreflectance measurement at several thousands points in 2 mm pitch on thesurface.

[0065] As a result, the deviation σ was 0.47 nm, and thus 3σ was 1.41nm. Accordingly, the thickness of the SOI layer is about 650 nm±1.41 nm,which shows significantly improved thickness uniformity of the SOIlayer, compared with the thickness uniformity of the conventional SOIwafer, the intended thickness ±3 nm.

Example 2

[0066] Two silicon mirror wafers of conductive type p having aresistivity of 20 Ω·cm and a diameter of 150 mm were prepared. Thesewafers were processed through Steps (a) to (f) shown in FIG. 1 todelaminate a portion of the bond wafer, and thereby provide SOI wafers.

[0067] The major process conditions for fabricating SOI wafers were asfollows.

[0068] 1) Thickness of the oxide film on the base wafer: 360 nm;

[0069] 2) Thickness of the oxide film on the bond wafer: 40 nm;

[0070] 3) Deviation in the thickness of the oxide film on the bondwafer: σ=0.20 nm;

[0071] 4) Condition of ion implantation: H⁺ ions, implantation energy:39 keV, implantation dose: 8×10¹⁶/cm²;

[0072] 5) Ion implantation depth: 340 nm (thickness of the SOIlayer+thickness of the oxide film on the bond wafer);

[0073] 6) Deviation in the ion implantation depth: σ=0.4 nm.

[0074] In Example 2, the deviation σ in the thickness of the oxide filmon the bond wafer is small as 0.20 nm, compared with the deviation inthe ion implantation depth (σ=0.4 nm). The thickness of the oxide filmon the bond wafer was measured in the similar manner to that ofExample 1. The conditions were set so that the thickness of the buriedoxide layer may be 400 nm.

[0075] The two wafers were stacked under the above condition (FIG.1(e)), and is subjected to the delamination heat treatment in a N₂ gasatmosphere, at 500° C. for 30 minutes (FIG. 1(f)).

[0076] The deviation in the thickness of the SOI layer of the SOI waferafter the delamination step was determined in the similar manner to thatof Example 1.

[0077] As a result, the deviation σ was 0.45 nm, and thus 3σ is 1.35 nm.Accordingly, the thickness of the SOI layer is about 300 nm±1.35 nm,which shows significantly improved thickness uniformity of the SOIlayer, compared with the thickness uniformity of the conventional SOIwafer, the intended thickness ±3 nm.

Example 3

[0078] Two silicon mirror wafers of conductive type p having aresistivity of 20 Ω·cm and a diameter of 150 mm were prepared. Thesewafers were processed through Steps (a) to (f) shown in FIG. 1 todelaminate a portion of the bond wafer, and thereby provide SOI wafers(provided that step (c) was omitted).

[0079] The major process conditions for fabricating SOI wafers were asfollows.

[0080] 1) Thickness of the oxide film on the base wafer: no oxide film;

[0081] 2) Thickness of the oxide film on the bond wafer: 50 nm;

[0082] 3) Deviation in the thickness of the oxide film on the bondwafer: σ=0.25 nm;

[0083] 4) Condition of ion implantation: H⁺ ions, implantation energy:20 keV, implantation dose: 8×10¹⁶/cm²;

[0084] 5) Ion implantation depth: 180 nm (thickness of the SOIlayer+thickness of the oxide film on the bond wafer);

[0085] 6) Deviation in the ion implantation depth: σ=0.4 nm.

[0086] In Example 3, the deviation σ in the thickness of the oxide filmon the bond wafer was small as 0.25 nm, compared with the deviation inthe ion implantation depth (σ=0.4 nm). Since the conditions were set sothat the thickness of the buried oxide layer may be 50 nm, the oxidefilm was not formed on the base wafer. The thickness of the oxide filmwas measured in the similar manner to that of Example 1.

[0087] The two wafers were stacked under the above condition (FIG.1(e)), and is subjected to the delamination heat treatment in a N₂ gasatmosphere, at 500° C. for 30 minutes (FIG. 1(f)).

[0088] The deviation in the thickness of the SOI layer of the SOI waferafter the delamination step were determined in the similar manner tothat of Example 1.

[0089] As a result, the deviation σ was 0.47 nm, and thus 3σ was 1.41nm. Accordingly, the thickness of the SOI layer was about 130 nm±1.41nm, which shows significantly improved thickness uniformity of the SOIlayer, compared with the thickness uniformity of the conventional SOIwafer, the intended thickness ±3 nm.

Comparative Example

[0090] Two silicon mirror wafers of conductive type p having aresistivity of 20 Ω·cm and a diameter of 150 mm were prepared. A portionof the bond wafer was delaminated to provide an SOI wafer according tothe conventional fabrication method shown in FIG. 3(B).

[0091] The major process conditions for fabricating SOI wafers were asfollows.

[0092] 1) Thickness of the oxide film on the base wafer: no oxide film;

[0093] 2) Thickness of the oxide film on the bond wafer: 400 nm;

[0094] 3) Deviation in the thickness of the oxide film on the bondwafer: σ=2.0 nm;

[0095] 4) Condition of ion implantation: H⁺ ions, implantation energy:80 keV, implantation dose: 8×10¹⁶/cm²;

[0096] 5) Ion implantation depth: 700 nm (thickness of the SOIlayer+thickness of the oxide film on the bond wafer);

[0097] 6) Deviation in the ion implantation depth: σ=0.4 nm.

[0098] In Comparative Example, the deviation σ in the thickness of theoxide film on the bond wafer was large as 2.0 nm, compared with thedeviation in the ion implantation depth (σ=0.4 nm). The thickness of theburied oxide film was 400 nm. The oxide film was formed only on the bondwafer, not on the base wafer. The thickness of the oxide film wasmeasured in the similar manner to that of Example 1.

[0099] The two wafers were stacked under the above condition (FIG.3(d)), and is subjected to the delamination heat treatment in a N₂ gasatmosphere, at 500° C. for 30 minutes (FIG. 3(e)).

[0100] The deviation in the thickness of the SOI layer of the SOI waferafter the delamination step were determined in the similar manner tothat of Example 1.

[0101] As a result, the deviation σ was 2.04 nm, and thus 3σ is 6.12 nm.Accordingly, the thickness of the SOI layer is about 300 nm±6.12 nm,which shows far inferior thickness uniformity of the SOI layer, comparedwith the intended thickness ±1.5 nm which is an expected value from thedeviation in the ion implantation depth σ=0.4 nm.

[0102] The present invention is not limited to the above-describedembodiment. The above-described embodiment is a mere example, and thosehaving substantially the same structure as that described in theappended claims and providing the similar action and effects are allincluded in the scope of the present invention.

[0103] For example, the process of fabricating the SOI wafer accordingto the present invention is not limited to that shown in FIG. 1. Otherprocesses such as cleaning, heat treatment or the like can be addedthereto. Furthermore, the order of the processes can be partly changedor omitted depending on the purpose.

[0104] Furthermore, in the above description, it is mainly explained asfor the case that the thickness of the oxide film formed on the bondwafer is made thin to make the deviation in the thickness of the oxidefilm formed on the bond wafer smaller than the deviation in the ionimplantation depth. However, the present invention is not limitedthereto. It is possible to make the deviation in the thickness of theoxide film smaller than the deviation in the ion implantation depth bymodifying other conditions than the thickness of the oxide film. Forexample, it is possible to make the deviation in the thickness of theoxide film smaller than the deviation in the ion implantation depth bymodifying other conditions for formation of the oxide film than thethickness of the oxide film.

What is claimed is:
 1. A method of fabricating an SOI wafer in which abond wafer to form a SOI layer and a base wafer to be a supportingsubstrate are prepared; an oxide film is formed on at least the bondwafer; hydrogen ions or rare gas ions are implanted in the bond wafervia the oxide film in order to form a fine bubble layer (enclosed layer)within the bond wafer; the ion-implanted surface is brought into closecontact with the surface of the base wafer; and then heat treatment isperformed to separate a thin film from the bond wafer with using thefine bubble layer as a delaminating plane to fabricate the SOI waferhaving an SOI layer; and wherein deviation in the thickness of the oxidefilm formed on the bond wafer is controlled to be smaller than thedeviation in the ion implantation depth.
 2. A method of fabricating anSOI wafer in which a bond wafer to form a SOI layer and a base wafer tobe a supporting substrate are prepared; an oxide film is formed on atleast the bond wafer; hydrogen ions or rare gas ions are implanted inthe bond wafer via the oxide film in order to form a fine bubble layer(enclosed layer) within the bond wafer; the ion-implanted surface isbrought into close contact with the surface of the base wafer; and thenheat treatment is performed to separate a thin film from the bond waferwith using the fine bubble layer as a delaminating plane to fabricatethe SOI wafer having an SOI layer; and wherein a thickness of the oxidefilm formed on the bond wafer is defined so that deviation in thethickness of the oxide film formed on the bond wafer may be smaller thanthe deviation in the ion implantation depth.
 3. The method offabricating an SOI wafer according to claim 1 wherein an oxide film ispreviously formed on the base wafer which is to be bonded to the bondwafer, and the thickness of the oxide film formed on the base wafer isdefined so that it can form the buried oxide layer with a desiredthickness in the SOI wafer together with the oxide film formed on thebond wafer.
 4. The method of fabricating an SOI wafer according to claim2 wherein an oxide film is previously formed on the base wafer which isto be bonded to the bond wafer, and the thickness of the oxide filmformed on the base wafer is defined so that it can form the buried oxidelayer with a desired thickness in the SOI wafer together with the oxidefilm formed on the bond wafer.
 5. The method of fabricating an SOI waferaccording to claim 1 wherein thickness of the oxide film formed on thebond wafer is 10 to 100 nm.
 6. The method of fabricating an SOI waferaccording to claim 2 wherein thickness of the oxide film formed on thebond wafer is 10 to 100 nm.
 7. The method of fabricating an SOI waferaccording to claim 3 wherein thickness of the oxide film formed on thebond wafer is 10 to 100 nm.
 8. The method of fabricating an SOI waferaccording to claim 4 wherein thickness of the oxide film formed on thebond wafer is 10 to 100 nm.
 9. An SOI wafer fabricated by the methodaccording to claim 1 .
 10. An SOI wafer fabricated by the methodaccording to claim 2 .
 11. An SOI wafer fabricated by the methodaccording to claim 3 .
 12. An SOI wafer fabricated by the methodaccording to claim 4 .
 13. An SOI wafer fabricated by the methodaccording to claim 5 .
 14. An SOI wafer fabricated by the methodaccording to claim 6 .
 15. An SOI wafer fabricated by the methodaccording to claim 7 .
 16. An SOI wafer fabricated by the methodaccording to claim 8 .
 17. An SOI wafer produced by bonding two waferswhich has a bonded surface in a buried oxide layer, or between theburied oxide layer and a base wafer, and thickness uniformity of an SOIlayer is ±1.5 nm or less.